This invention relates to the polymerisation of siloxanes catalyzed by certain phosphazene bases, and in particular to the formation of polymeric siloxanes having amino-functionality.
In EP0860461-A, there is described a process for the ring-opening polymerization of cyclosiloxanes, which comprises contacting a cyclosiloxane with 1 to 500 ppm of a phosphazene base, by weight of cyclosiloxane, in the presence of water. In GB 2311994, there is described a method of effecting polycondensation which comprises contacting at a temperature of from 0 to 200xc2x0 C. and a pressure up to 350 torr, a silanol-containing organosiloxane with an amount of a peralkylated phosphazene base which is effective for polycondensation of said organosiloxane. The preferred peralkylated phosphazene base has the formula 
The prior art is useful for the manufacture of higher molecular weight polysiloxane materials with hydrocarbon or hydroxyl substituents. There is a need for making siloxane polymers which have other functionalities, and in particular amine functionality. It is particularly difficult to make amino-functional siloxanes by polymerization. An existing method uses equilibration of cyclic siloxanes with aminofunctional silanes or siloxanes in the presence of strong base catalysts, such as potassium hydroxide or potassium silanolate, described in EP 575972. Alternatively, a condensation reaction is used starting from silanol-functional siloxane polymers in conjunction with amino-functional organosilicon compounds, e.g. silanes. This method is useful and effective in many ways, but slow, and often requires a complex catalyst system. Many catalytic systems are affected by the presence of amines, and are thus not suitable as a solution to the problem.
This invention is a polymerization process comprising mixing a siloxane polymer with an organosilicon compound having at least one silicon-bonded group RN, which is a substituent comprising at least one amine group, with a phosphazene base catalyst and allowing the siloxane and organosilicon compound to polymerize to form amino-functional polyorganosiloxane polymers.
We have surprisingly found that phosphazene base materials are effective catalysts for polymerization of siloxanes in order to provide amino-functional siloxanes. They are furthermore found to be effective, whether they are used to make the amino-functional siloxanes via condensation or equilibration reaction, or even, if desired, by a combination of both reaction types.
According to a first aspect of the invention there is provided a polymerization process comprising mixing certain siloxanes and organosilicon compounds having at least one silicon-bonded group RN, which is a substituent comprising at least one amine group, with one or more phosphazene base catalysts and allowing the siloxanes and organosilicon compounds to polymerize to form amino-functional polyorganosiloxane polymers.
Phosphazene bases themselves are known to be extremely strong bases. Numerous phosphazene bases, some of which are ionic phosphazene bases and routes for their synthesis have been described in the literature, for example in Schwesinger et al., Liebigs Ann. 1996, 1055-1081.
Phosphazene bases are found to be a very powerful catalyst for polymerization of siloxane materials, and can therefore be present in a relatively low proportion, for example from 1 to 2000 ppm, preferably 2 to 1000 ppm by weight, based on the weight of siloxanes. The proportion of catalyst actually used will be selected depending on the speed of polymerization that is sought or on the size of polymer required.
A proportion of water may be present in the reaction, especially where the phosphazene used is a non-ionic phosphazene. Where this is the case, it is preferably at least 0.5, more preferably from 0.5-10 mols per mol of the ionic phosphazene base, most preferably from 1 to 10 mols per mol of ionic phosphazene base. It is possible to allow higher proportions of water, and this can have the benefit of enabling greater control over the polymerization reaction, as described in more detail below.
In principle, any phosphazene base is suitable for use in the present invention. Phosphazene bases generally include the following core structure Pxe2x95x90Nxe2x80x94Pxe2x95x90N, in which free N valencies are linked to hydrogen, hydrocarbon, xe2x80x94Pxe2x95x90N or xe2x95x90Pxe2x80x94N, and free P valencies are linked to xe2x80x94N or xe2x95x90N. Some ionic phosphazene bases, for example 1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis{tris(dimethylamino)-phosphoranylidenamino}-2xcex5, 4xcex5-catenadi(phosphazene)}, are commercially available e.g. from Fluka Chemie AG, Switzerland. The ionic phosphazene bases preferably have at least 3 P-atoms. Some preferred phosphazene bases are of the following general formulae:
((R12N)3Pxe2x95x90Nxe2x80x94)x(R12N)3xe2x88x92xPxe2x95x90NR2
{((R12N)3Pxe2x95x90Nxe2x80x94)x(R12N)3xe2x88x92xPxe2x80x94N(H)R2}+{Axe2x88x92}
{((R12N)3Pxe2x95x90Nxe2x80x94)y(R12N)4xe2x88x92yP}+{A}xe2x88x92
{(R12N)3Pxe2x95x90Nxe2x80x94(P(NR12)2xe2x95x90N)nxe2x80x94P+(NR12)3}{Axe2x88x92}
in which R1, which may be the same or different in each position, is hydrogen or an optionally substituted hydrocarbon group, preferably a C1-C4 alkyl group, or in which two R1 groups bonded to the same N atom may be linked to complete a heterocyclic ring, preferably a 5- or 6-membered ring; R2 is hydrogen or an optionally substituted hydrocarbon group, preferably a C1-C20 alkyl group, more preferably a C1-C10 alkyl group; x is 1, 2 or 3, preferably 2 or 3; y is 1, 2, 3 or 4, preferably 2, 3 or 4; n is an integer with a value of from 1 to 10.and A is an anion, preferably fluoride, hydroxide, silanolate, alkoxide, carbonate or bicarbonate.
Particularly suitable compounds are those where R1 is methyl, R2 is tertiary butyl or tertiary octyl, x is 3, y is 4, n is 1 to 4 and A is fluoride or hydroxide. Suitable phosphazene base catalysts are commercially available, or can be made by a process disclosed by Schwesinger et al, as indicated above. The compounds of the formula {(R12N)3Pxe2x95x90Nxe2x80x94(P(NR12)2xe2x95x90N)zxe2x80x94P+(NR12)3} {A}xe2x88x92 may be made by a method which comprises reacting a linear phosphonitrile halide compound, preferably chloride, with a compound selected from a secondary amine, a metal amide and a quaternary ammonium halide to form an aminated phosphazene material, followed by an ion exchange reaction replacing the anion with a nucleophile. Phosphonitrile halide compounds and methods of making them are well known in the art; for example, one particularly useful method includes the reaction of PCl5 with NH4Cl in the presence of a suitable solvent. Secondary amines are the preferred reagent for reaction with the phosphonitrile halide, and a suitable secondary amine has the formula R3 2NH, wherein R3 is a hydrocarbon group having up to 10 carbon atoms, or both R3 groups form a heterocyclic group with the nitrogen atom, for example a pyrollidine group, a pyrrole group or a pyridine group. Preferably, R3 is a lower alkyl group, more preferably a methyl group, or both R3 groups form a pyrollidine ring. Suitable preferred secondary amines include dimethylamine, diethylamine, dipropylamine and pyrollidine. Preferably the reaction is carried out in the presence of a material which is able to capture the exchanged halides, e.g. an amine such as triethylamine. The resulting by-product (e.g. triethyl ammonium chloride) can then be removed from the reaction mixture, e.g. by filtration. The reaction may be carried out in the presence of a suitable solvent for the phosphonitrile chloride and linear phosphazene base. Suitable solvents include aromatic solvents such as toluene. The linear phosphazene material which is formed this way can be passed through an ion exchange reaction (preferably an ion exchange resin) whereby the anion is replaced with a hard nucleophile, preferably hydroxyl or alkoxy, most preferably hydroxyl. The phosphazene is preferably dispersed in a suitable medium prior to passing through an ion exchange system. Suitable media include water, alcohol and mixtures thereof.
The polymerization can be carried out in bulk or in the presence of a solvent. Suitable solvents are liquid hydrocarbons or silicone fluids. The phosphazene base catalyst can be diluted in a hydrocarbon solvent, such as hexane, heptane or toluene, or dispersed in a silicone fluid such as polydiorganosiloxanes. Where the phosphazene base catalyst is initially in a solvent such as hexane, the hexane can be removed by evaporation under vacuum, and the catalyst dispersed in a silicone fluid to give a stable clear solution. When this silicone-dissolved catalyst is used for polymerization reactions, the catalyst disperses evenly and gives reproducible results. The catalyst can also be dissolved in water, and this has the advantage of moderating and enabling greater control over the polymerization reaction, as described below.
The polymerization reaction can be carried out under heating, for example to 100xc2x0 C. or higher, which is appropriate when the catalyst activity has been moderated as described below. The method of the invention may conveniently be carried out at room temperature. The temperature may also be as high as 250xc2x0 C. Preferably, however, the temperature range is from 20 to 170xc2x0 C., most preferably from 50 to 170xc2x0 C. The time taken for polymerization will depend on the activity of the catalyst in the chosen system, and on the desired polymer product. In the absence of moderation, the phosphazene base catalysts are sufficiently active to convert siloxanes to high molecular weight amino-functional polysiloxane materials within a short time frame.
One of the starting materials for the condensation reaction is siloxane polymers having silicon-bonded hydroxyl groups or hydrolyzable groups such as alkoxy or aryloxy groups, which may form silanol groups in situ. These include, for example, organosiloxanes having the general formula (3): 
In formula (3), R3 is as hydrogen or an alkyl or aryl group having up to 8 carbon atoms, each R4 is the same or different and denotes a monovalent hydrocarbon group preferably having 1 to 18 carbon atoms or halogenated hydrocarbon group preferably having 1 to 18 carbon atoms and t is an integer having a value of from at least 2. Preferably R4 denotes an alkyl group having from 1 to 6 carbon atoms and more preferably a methyl group. The value of t is preferably such that the average viscosity of the organosiloxanes does not exceed 200 mm2/s at 25xc2x0 C.
Suitable organosiloxanes may have silicon-bonded Rxe2x80x2 groups, where Rxe2x80x2 is selected from a hydroxyl group and a hydrolyzable group, where the Rxe2x80x2 groups are in the polymer chain, but preferably the Rxe2x80x2 groups are present in end-groups. Organosiloxanes having terminal silicon-bonded hydroxyl groups are well known in the art and are commercially available. They can be made by techniques known in the art, for example, by hydrolysis of a chlorosilane, separation of the linear and cyclic material produced by the hydrolysis, and subsequently polymerizing the linear material. Preferably suitable organosiloxanes have one silicon-bonded hydroxyl group in each terminal group and have at least 80% of the R4 groups denote a methyl group. Suitable organosiloxanes for use as reagents in a polymerization process in which the ionic phosphazene catalysts are used include organosiloxanes having terminal hydroxydiorganosiloxane units, e.g. hydroxyldimethyl siloxane end-blocked polydimethylsiloxanes, hydroxyldimethyl siloxane end-blocked polydimethyl poly-methylphenyl siloxane copolymers.
Another ingredient for the polymerization reaction by condensation is an amino-functional organosilicon compound, which has a silicon-bonded hydroxyl group or hydrolyzable group present. An example of a suitable organosilicon compound is a silane of the general formula RNxe2x80x94Si (R)zxe2x80x94Rxe2x80x23xe2x88x92z, wherein RN and Rxe2x80x2 are as defined above, R denotes a hydrocarbon group having up to 20 carbon atoms and z has a value of 1 or 2. Another example is a siloxane having at least one silicon-bonded group Rxe2x80x2 and one silicon-bonded group RN. Examples of suitable groups RN include xe2x80x94(CH2)3NHC6H5, xe2x80x94(CH2)3NH2, xe2x80x94CH2.CH(CH3)CH2NH2, xe2x80x94(CH2)3NH(CH2)2NH2, xe2x80x94CH2CH(CH3)CH2NH(CH2)2NH2, 
For the polymerization reaction which uses equilibration, cyclic or linear siloxanes which do not have the above mentioned silicon-substituted Rxe2x80x2 groups are suitable. Suitable cyclosiloxanes, also known as a cyclic siloxanes, are well known and commercially available materials. They have the general formula (R22SiO)n, wherein R2 is as defined above, and preferably denotes hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group having up to 8 carbon atoms, n denotes an integer with a value of from 3 to 12. R2 can be substituted, e.g. by halogen such as fluorine or chlorine. The alkyl group can be, for example, methyl, ethyl, n-propyl, trifluoropropyl, n-butyl, sec-butyl, and tertiary-butyl. The alkenyl group can be, for example, vinyl, allyl, propenyl, butenyl and hexenyl. The aryl and aralkyl groups can be, for example, phenyl, tolyl, and benzoyl. The preferred groups are methyl, ethyl, phenyl, vinyl, and trifluoropropyl. Preferably at least 80% of all R2 groups are methyl or phenyl groups, most preferably methyl. It is most preferred that substantially all R2 groups are methyl groups. Preferably the value of n is from 3 to 6, most preferably 4 or 5. Examples of suitable cyclic siloxanes are octamethyl cyclotetrasiloxane, decamethyl cyclopentasiloxane, cyclopenta (methylvinyl) siloxane, cyclotetra (phenylmethyl) siloxane and cyclopenta methylhydrosiloxane. One particularly suitable commercially available material is a mixture of octamethylcyclo-tetrasiloxane and decamethylcyclopentasiloxane.
The starting material for equilibration polymerization can be instead of or in addition to cyclic siloxanes as described above, any organosiloxane material having units of the general formula R5aSiO4xe2x88x92a/2 wherein R5 denotes a hydrogen atom, a hydrocarbon group having from 1 to 18 carbon atoms, a substituted hydrocarbon group having from 1 to 18 carbon atoms or a hydrocarbonoxy group having up to 18 carbon atoms and a is as above defined but preferably has on average a value of from 1 to 3, preferably 1.8 to 2.2. Preferably the organosiloxanes are dialkylsiloxanes, and most preferably dimethylsiloxanes. They are preferably substantially linear materials, which are end-blocked with a siloxane group of the formula R53SiO1/2, wherein R5 is Rxe2x80x2.
Sources for RN substituents for the polymers to be made by equilibration include cyclic siloxanes which have at least one RN substituent present on a silicon atom, linear siloxane materials which do not have a group Rxe2x80x2 linked to a silicon atom, but which do have at least one silicon-bonded RN group, preferably on a terminal silicon atom of the siloxane polymer.
It is even possible to provide the source of RN groups for the polymers to be made by the process of the present invention in so-called end-blockers. These may be used to control the chain length of any polymers made, and if they contain the RN group, they will be used at the same time to functionalize the polymers with an amine-containing substituent.
Suitable end-blockers for the equilibration polymers to be formed, include polysiloxanes in the molecular weight range from 160 upwards, in particular polydimethylsiloxanes of the general formula MDxM where M is trimethylsilyl, D is xe2x80x94Si(CH3)2Oxe2x80x94 and x has a value of from 0 to 20. The end-blocker may have one or more functional groups such as hydroxyl, vinyl or hydrogen. Suitable ingredients for end-blocking when using a condensation reaction include short chain polymers e.g. organosiloxanes having only 1 group Rxe2x80x2 and Rxe2x80x2Si containing silanes. In the absence of added end groups providing ingredients used in the process according to the invention, the molecular weight is determined by the catalyst concentration. An ingredient providing end-blocker groups may be added in a proportion calculated to produce a desired molecular weight of polymer. Water also acts as a end-blocker, with the introduction of hydroxyl functional groups.
We have also surprisingly found that phosphazene base materials are effective as catalysts for the combined polymerization via condensation and polymerization by equilibration, when carried out simultaneously. This is unexpected as there is usually a substantial difference in catalytic rate between both reactions. The speed of polymerization via equilibration seems normally to be substantially faster than for the condensation reaction described in this application. It was therefore surprising to find that the same catalyst can be used for combined polymerization via condensation and equilibration by mere mixture of the siloxane materials used for condensation polymerization, with cyclic siloxanes or certain linear siloxanes as described below, which are suitable for polymerization by equilibration. The combined reaction did not seem to favor one polymerization reaction to the detriment of the other.
By using a combination of condensation and equilibration reactions, it is possible to arrange the reagent mixture in a way to control the end-product, for example by controlling the amount and type of ingredients which are used to cause end-blocking, by varying the ratio of siloxanes which polymerize via condensation to siloxanes which polymerize via equilibration.
Thus the process according to the invention will be useful for making amino-functional polyorganosiloxanes having at least one silicon-bonded group RN and having units of the general formula Rxe2x80x3aSiO4xe2x88x92a/2 (2) wherein Rxe2x80x3 is hydroxyl or a hydrocarbon and a has a value of from 0 to 3. Preferably at least 80% of all Rxe2x80x3 groups are alkyl or aryl groups, more preferably methyl groups. Most preferably substantially all Rxe2x80x3 groups are alkyl or aryl groups, especially methyl groups. The polyorganosiloxanes are preferably those in which the value of a is 2 for practically all units, except for the end-blocking units, and the siloxanes are substantially linear polymers of the general formula Rxe2x80x3(Rxe2x80x32SiO)pSiRxe2x80x33. (3) wherein Rxe2x80x3 is as defined above and p is an integer. It is, however, also possible that small amounts of units wherein the value of a denotes 0 or 1 are present. Polymers with such units in the chain would have a small amount of branching present. Preferably Rxe2x80x3 denotes a hydroxyl group or an alkyl or aryl group, e.g. methyl or phenyl. The viscosity of the polyorganosiloxanes which may be produced by the process using a catalyst according to the present invention may be in the range of from 1000 to many millions mm2/s at 25xc2x0 C., depending on the reaction conditions and raw materials used in the method of the invention.
The process according to the invention can be used to make a whole range of amino-functional polyorganosiloxanes, including liquid polymers and gums of high molecular weight, for example from 1xc3x97106 to 100xc3x97106 amu. The molecular weight of polyorganosiloxanes is effected by the concentration of materials used in the reaction, which will provide end groups. The catalyst used in the present invention has sufficient activity to enable the formation of polymers in a reasonable time at a low catalyst concentration.
When the desired polymer has been formed, it is usually desirable to neutralize the catalyst to stabilize the product and prevent any further reaction. Suitable neutralizing agents are acids such as acetic acid, silyl phosphate, polyacrylic acid chlorine substituted silanes, silyl phosphonate or carbon dioxide.
We have found that air reacts very rapidly with the catalyst solutions giving a hazy material which eventually leads to an insoluble liquid phase. This is believed to be due to the reaction of the catalyst with CO2 to form a carbonate salt. We have also found that this deactivation of the catalyst can be reversed e.g. by heating, purging with inert gas or subjecting the mixture to reduced pressure. This makes it possible to moderate or control the polymerization reaction. This is particularly advantageous in view of the very rapid reaction which occurs when the catalyst is not moderated. Because of the low levels of catalyst employed in these reactions (which can be as low as 100 to 2000 ppm), the reaction with CO2 needs to be taken into account to control the reaction and obtain reproducible results. By dissolving the phosphazene base in water, in which it is very soluble and very stable, the catalyst activity becomes much more controllable and the polymers produced are of lower molecular weight. This is caused by the water acting as a catalyst inhibitor and also as an end-blocker. The inhibiting effect of the water can be reduced by reducing the amount of water present e.g. by heating. At temperatures below 100xc2x0 C. the rate of polymerization is relatively slow in the presence of water and/or CO2, for example taking up to more than 24 hours to reach gum viscosity. At temperatures above 100xc2x0 C. (e.g. 100-150xc2x0 C.), polymerization becomes much faster, for example taking up to 5-60 minutes to reach gum viscosity. Such control of the reaction can also be achieved if the water is mixed with or replaced by alcohol (e.g. C1-C6 alcohols such as methanol or ethanol).
We have also found that polymerization can be prevented by exposing a mixture of siloxanes and phosphazene base catalyst to air and/or CO2 or larger amounts of water. The polymerization can then be initiated (xe2x80x9ccommand polymerizationxe2x80x9d) simply by removing the air and/or CO2 or water e.g. by heating the mixture (e.g. to 100xc2x0 C. to 170xc2x0 C. for a few minutes). A D4 catalyst mixture (2 to 50 ppm of catalyst) is stable in air at 20xc2x0 C. for extended periods (up to 7 days).